Crowd estimation - Guinness World Record - help!

Hi guys
 
It's a lovely Friday afternoon here, and I took myself and my computer and Donagh up to Achill yesterday to work from here, as the weather forecast is good.  I'm looking out the window here as I type, and it's fabulous.  The sea looks so blue, and the colours of the yellow gorse, purple heathers and the rusty brown of the mountains are so vivid.  I'm looking forward to a quiet weekend and recharging the batteries.  Donagh actually slept ;ast night from about 10pm to 7.30am, a real record! 
 
Anyway, I digress.  Our annual flag day in Galway takes place on Monday 7th June, which is the bank holiday, and it also co-incides with the Airshow, with the Red Arrows flying overhead doing their acrobatics, and it also co-incides with the Irish leg of the International Powerboat Races, with the rich boys racing their large toys off the Prom in Salthill, so it should be a fantastic day out!  The last Airshow in 2008 had a crowd of 100,000, so at least that number should be there again on the day, with the added attraction of the Powerboats.  So, our Garda permit covers the city centre and Salthill, so we should do really well if we can get the numbers of volunteers mobilised.
 
However, not being content with this, I also see the opportunity to do a big event, so I am hoping to set a new world record for the largest number of people Irish dancing, or the biggest ceili in the world, and get as many of the 100,000 crowd to do a 'Walls of Limerick' or something like this for 5 minutes.  The only remaining obscacle is how to count the numbers.  Guinness had suggested a log book with every person signing to say they had participated, and this being signed off by two independent witnesses, or else having supervisors for every 50 participants, to confirm that they had witnessed 50 people dancing.  But with everyone moving around, this is like to be very difficult not to double count the numbers.  So I think a technological solution, using GIS or some application which can either sense the people, or can count the people from scanning an aerial photo, is the way to go.  So if anyone has any ideas or knows of any companies doing stuff like this, please let me know!  If we can pull this off, we should be able to raise €100,000 on the day.
 
Have a lovely weekend!
Lots of love
Nicky